[libvirt] [PATCH] fix detach-disk for inactive xen domains

detach-disk was not working for inactive xen domains due to lookup of device ID from xenstore. When detaching disks there is no need to use device id as device name can be used directly. Unfortunately that same is not true for network devices. In fact, AFAICT, there is no way to "hot unplug" a network device from inactive domain using xen tools. I have tested attaching/detaching disks to/from both active and inactive domains. I've also ensured no regressions in attaching/detaching network devices on active domains and attaching network devices on inactive domains. As noted above, detaching network devices from inactive domains does not work, but I'm not sure what we can do about that since xen tools themselves don't support this. Any suggestions are kindly welcomed. Oh, and I also ensured all of these scenarios work with both PV and HVM domains. Regards, Jim